1. Field of the Invention
The present invention relates to a method and an apparatus for use in a borehole having fluid therein, and designed to carry out measurements related to said fluid, such as the flow rate and velocity. More particularly, the invention pertains to the kind of logging tool called "packer flowmeter", wherein the fluid is diverted into a measuring pipe including a flowmeter device which is of reduced cross section with respect to the casing.
2. Description of the Related Art
As known in the art of logging techniques, a well which has been determined to be promising for oil production, is provided with a metallic casing, and cement is injected between the earth formation and the casing. Perforations are then made through the casing/cement and in the oil productive formation, so as to allow oil to flow up to the surface through a tubing beforehand arranged in the well coaxially to the casing. Among other information useful to the user, is the amount of oil produced. The invention may also be used, besides the production wells hereabove referred to, in the so-called "injection wells" in which a high pressure fluid is injected in a first well so as to make the oil move in earth formation towards a neighboring second well which will become the oil producing well.
With respect to the two above mentioned well categories, there have been developed over the years different logging tools and techniques for measuring among others, fluid flow velocity and/or flow rate. These known tools comprise a body centered in the borehole and including a flowmeter device (such as a spinner). Two families of such tools are currently available. In the first kind, the flow is measured through the full diameter of the borehole wall or casing. In the second kind, called "packer flowmeter" and usually designed for low flow velocities, the flow is diverted into a measuring pipe of reduced diameter by a cone-shaped packer device which is designed to close off the annulus between the outside diameter of the tool and the borehole wall or the casing. Commonly used packers are made, either of an inflated cloth bag disposed inside a cage made of longitudinal metal blades, or of a metal basket. Examples of such known packers are described e.g. in U.S. Pat. Nos. 2,649,710; 2,706,406; 3,176,511; 4,552,076; 4,566,317; or 4,800,752.
It is of importance that all the fluid flowing in the casing be diverted into the measuring pipe, so that the measurement is accurate In other words, the sealing between the casing and the terminal bottom part of the packer is critical.
In this respect, it has been proposed, as shown in U.S. Pat. No. 4,800,752 issued to George Ed. Piers, to provide the packer with an inflatable tube disposed at the bottom end of said packer. Said tube is inflated by pumping fluid therein, and thus forms a ring.
Although the foregoing known packer flowmeters have been operated relatively satisfactorily up to now, they can be improved.
Firstly, with respect to the '752 patent device, the additional inflatable tube requires a pump, which increases the complexity of the tool.
Secondly, in multiphase flow, which is the most common case in an oil productive borehole, the water phase, at the exit of the measuring pipe, because of gravity, separates from the oil phase and falls down in the annulus situated above the packer. Accordingly, the upper side (or downstream side) of the packer is submitted to a higher pressure than the bottom side (or upstream side) of the same. This is detrimental since it may cause the packer to collapse down, at least locally, and thus create a substantial leak in the sealing.
Furthermore, the design of the terminal part of the packer in contact with the casing is critical. Since the packer is attached to the support metal blades by this very terminal part, it has to be strong. However, the sealing requires the terminal part to be as soft as possible in order to match the form of the casing. These two opposite requirements lead to a non satisfactory compromise.
According to the above, there is a need for a logging flowmeter tool provided with a reliable sealing between the packer and the casing.